Cooling systems for both industrial plants and utilities are subject to fouling by Asiatic clams whether the system is using cooling water on a once-through basis or of the recirculating type. The once-through systems operate by drawing cooling water through the process to be cooled on a one-time basis and discharge the water directly to the receiving body with a short residence time (usually minutes to hours) whereas recirculation cooling systems require the addition of only a fraction of the system volume as makeup water. Additionally, the service water systems (water, safety, and auxiliary cooling) which are often a part of these cooling systems are also quite vulnerable to macroinvertebrate fouling, primarily because they do not run continuously, and the conduits are of a smaller diameter.
The extent and type of Asiatic clam fouling will depend upon many factors such as the source of the cooling water, the season, the water temperature, the growth rate of the Asiatic clam, and the linear velocity of the cooling water. Because of the large quantities of cooling water used, the locality of the plant will dictate the water's source. A fresh water cooling system will be drawing from a river, lake or well, whereas plants situated along coastal areas will most likely utilize brackish or marine water for their cooling purposes.
Both once-through and recirculating types of cooling water are treated prior to entering the system by screening to remove objects which are large enough that they could damage pumps and heat exchange equipment. This screening does not prevent the passage of the early (microscopic) life-stages or larval stages of the Asiatic clams, which are the precursors to fouling as growth conditions are usually favorable within these systems. These early life stages of the Asiatic clams will settle out in low flow areas within the cooling system and grow and accumulate to a fouling size.
As a specific example of how Asiatic clams can cause fouling problems, a description of some characteristics are as follows:
One-year-old clams are capable of plugging valves and nozzles. Two-year-old clams can cause mechanical damage to impellers and other moving parts of water-distribution systems. At six years, the clam can damage tires of construction vehicles. As in all other clams, growth is rapid in early years and then tapers off. "Clams--A Growing Threat to Implant Water Systems", Plant Engineering, June, 1979, p. 165.
The Asiatic clams are very tolerant of many chemicals and often occur in great abundance. They have accumulated to depths of two meters in the Delta-Mendota Canal in California and have caused severe reduction in water flow. Some industrial plants have had difficulty obtaining fire insurance after inspectors found the fire protection systems plugged with Corbicula shells. Pump impellers have been damaged by shells in some industrial plants. The number of power plants which have experienced problems with Asiatic clams has been steadily increasing during the past several years. Problems in fossil-fueled power plants most often relate to pluggage of condenser tubes, surface water heat exchangers, and blockage of fire protection systems. In addition to these problems, nuclear power plants may have other problems associated with the shutdown service water and emergency reactor cooling systems. For further information, see also Mattice, J. S., 1979, "Interactions of Corbicula sp with Power Plants", pages 119-138 and Goss, L. B. et al., 1979, "Control Studies on Corbicula for Steam Electric Generating Plants", pages 139-151, in J. C. Britton (ed), Processing, First International Corbicula Symposium, Texas Christian University Research Foundation, Fort Worth, Tex., 313 pages.
Another threat to cooling water systems is Dreissena spp, the Zebra mussel, which has caused fouling problems in Europe and more recently, in North America. Morphologically, Asiatic clams are different from Zebra mussels. Asiatic clams in the larval stage are brooded in adult clam's gills and are not free swimming, while Zebra mussels in the larval stage are free swimming veligers. This allows Zebra mussel veligers to swim freely throughout any aqueous system they are present in and allows them to attach, by way of byssal threads, to any hard surface they come in contact with. As such, clustering of Zebra mussels can occur on these surfaces as they will even attach to other Zebra mussel shells. This formation of clusters will foul aqueous systems where they become situated.
Asiatic clams by their nature are burrowers which prefer silt and sand banks in which to live. Asiatic clams lack the ability to attach to substrates found in aqueous systems and fouling is generally the result of clogging of intake valves by the shells of the individual clams, whereas Zebra mussels will attach to all components in an aqueous system. This difference in morphology also dictates the type of treatment which is effective at controlling their growth. Given their spawning pattern and relatively non-mobile nature, Asiatic clams are effectively treated with a shock or one-time treatment of inhibitor chemicals. Zebra mussels are best treated by a continuous feed of chemicals over an extended period of time.
Fouling control of Asiatic clams has been attempted using physical/mechanical and chemical techniques (see, e.g., U.S. Pat. No. 4,328,638), but no foolproof combination has been developed. For example, chlorine, which has been by far the most used biofouling control agent, has several limitations: prolonged exposure times are required to achieve efficacy, chlorine demand of the cooling water reduces its potency, and strict environmental regulations are being imposed which act to severely limit the discharge of chlorine residues, and in some cases seek to eliminate its use altogether.
In addition to chlorine, Smith, U.S. Pat. No. 4,462,914 discloses the use of a high density cationic polymer to control Corbicula. While the polymer appears to be efficacious toward the adult clam after a six day exposure period, it suffers from some of the same drawbacks as chlorine.
The above-mentioned concerns over potential fouling biocides is well described by the following excerpt from the December 1983 Proceedings of the Electric Power Research Institute Symposium:
"Chemical controls have an inherent liability. What can kill inside the power plant may also impact the receiving water body: chemical toxicants are not specific. The perfect chemical would be stable enough to be effective inside the plant, but become nontoxic, via chemical reaction or decay, before or as it entered the receiving water body. So far, no chemical meets these specifications: chlorine and bisulfate/sulfide which have actually been used in an attempt to control Corbicula fouling have not been effective alone or have been successful only under limited conditions. Such a chemical may not exist, but scheduling of application of a chemical at the beginning of scheduled outages may offer a less stringent alternative, because of the possibility of extending holdup times."
U.S. Pat. No. 4,561,983 discloses the use of a nitrostyrene compound to control the fouling potential of Asiatic clams. U.S. Pat. No. 4,579,665 discloses the use of a nitrostyrene compound and an alkyl thiocyanate compound to control Asiatic clam fouling potential. Also, the compound of the present invention has been utilized for control of microorganisms, i.e., bacteria, fungi, and algae, but has not been suggested for control of larger, more complex organisms, especially macroinvertebrates, in aqueous systems. See, e.g., U.S. Pat. Nos. 2,906,595 and 3,142,615, and 21 CFR 176.300. Bacteria, fungi and algae microorganisms are dependent upon the presence of or absence of macroinvertebrates, such as mollusks, is essentially independent of the presence of metabolizable components in the water because they are much more complex organisms than microorganisms, both in terms of anatomic and physiological complexity and position in the food chain. Asiatic clams are unable to exist on metabolizable components. Rather, clams require microscopic bacteria and algae as foodstuff. Until the unexpected discovery of the applicants, the use of the compound of the present invention has never before been appreciated to control Asiatic clams.
U.S. Pat. No. 5,118,346 teaches the use of quaternary ammonium and phosphonium salts with fluoborate and fluophosphate anions formulated in paint to form antifouling paint films. These films are particularly effective at protecting objects immersed in seawater which are often covered with attached organisms such as mussels, barnacles and hydrozoans.
European patent application EP 0549 006 A2 teaches the use of ammonium and phosphonium salts including alkyl phosphonium chloride salts as biocides in general. Amongst the organisms which these salts are effective against are mussels. However, there is no teaching or discussion as to their effects against Asiatic clams. Further, these salts are taught to be used on or with solid or porous technical materials where they attack organisms such as mussels which attach to the solids.